Conventionally, as one of the welding means for metallic members, spot welding in which by supplying current with the metallic members press-fit to each other, the metallic members are melted due to resistant heat and joined together has been widely utilized. In this spot welding, as shown in FIG. 12, a welding gun in which an electrode chip 95 is fit to a tapered portion 90a formed at tip of the shank 90 is mounted on a robot arm. During use in a long period, the shape of the tip of the electrode chip 95 is deformed and worn thereby making it difficult to secure a welding quality. Thus, when a predetermined processing time or a predetermined quantity of welding points is exceeded, the electrode chip 95 is removed from tip of the shank 90 and replaced with a new product or a processed one by polishing so as to secure the shape of the tip of the electrode chip 95.
Recently, most welding object members are treated in antirust processing with chemical processing agent, and therefore, there is a tendency that the electrode chip 95 is worn rapidly thereby increasing the replacement frequency. However, replacement of the electrode chip 95 by stopping production line each time reduces production efficiency. Accordingly, there has been proposed an automatic removing unit for the electrode chip 95 in order to replace the electrode chip 95 without necessity of stopping the production line, as shown in patent document 1.
However, in the automatic removing unit of the conventional electrode chip 95 described in this patent document 1, a piece of a fork-like lever is inserted into the tapered portion 90a of the shank 90 and by tilting this lever while butting it against the shank 90 side, the electrode chip 95 is almost forcibly removed by application of the principle of the leverage. This case has a problem that a concentrated load is applied to the shank 90 and a corner of the electrode chip 95 which the lever makes contact with and thus they are likely to be damaged. Another problem is that the amount of insertion of the lever into the tapered portion 90a of the shank 90 is not stabilized thereby producing a removal failure of the electrode chip 95.
Then, an electrode chip removal device as shown in patent document 2 has been proposed. This electrode chip removal device is constituted of a fixed pawl and a movable pawl which opens/closes vertically and a U-shaped insertion portion for introducing the shank is formed in the fixed pawl and movable pawl. The tapered portion 90a of the shank 90 on which the electrode chip 95 is mounted is inserted into the insertion portion of the fixed pawl and the movable pawl and by moving the movable pawl, the fixed pawl and the movable pawl are moved apart from each other so as to remove the electrode chip 95 from the tapered portion 90a of the shank 90.
However, the electrode chip 95 may be sometimes melted and coagulated with a welding object member and if the shank 90 is separated in this condition, the shank 90 can be bent slightly. Further, if the shank 90 strikes a jig or a welding object member, the shank 90 may be bent. If the shank 90 is bent in such a way, sometimes the shank 90 cannot be inserted into the insertion portion of the fixed pawl and the movable pawl and if it is attempted to insert it forcibly, sometimes the shank 90 is damaged.    Patent document 1: Japanese Patent Application Laid-Open No. 2002-79382    Patent document 2: Japanese Patent Application Laid-Open No. 2002-859